1. Field of the Invention
The present invention relates to a field-effect transistor formed of a group-III nitride semiconductor for a power switching transistor which is used, for example, in a general-purpose inverter.
2. Description of the Prior Art
A GaN based nitride semiconductor has a wider band gap, a larger dielectric breakdown electric field, and a higher saturation drift velocity than those of, for example, a Si semiconductor or a compound semiconductor such as GaAs. Therefore, the GaN based nitride semiconductor is a subject of interest for application in a high voltage power device and an electronic device such as a high-speed, high-power transistor. Especially, a heterojunction field-effect transistor (HFET) of AlGaN/GaN having a large drain current has been reported (see Y. F. Wu et al., Appl. Phys. Lett., 69 (1996) 1438), wherein a major characteristic of the heterojunction field-effect transistor of AlGaN/GaN is that in an AlGaN/GaN heterojunction generally formed on a (0001) plane, due to effects of polarization, a high sheet carrier concentration of 1013 cm−2 or higher can be obtained even in an undoped state. As an application field for advantageously using the feature of a large current operation capability of a GaN based electronic device, a power switching transistor is thought to be promising. However, for practical use, it is highly required to realize a high breakdown voltage and to realize a normally OFF state in which a current does not flow when a voltage is not applied to a gate electrode.
Conventionally, an AlGaN/GaN HFET structure has been disclosed in Japanese Laid-Open Patent Publication No. 2003-109973 and in Y. F. Wu et al., Appl. Phys. Lett., 69 (1996) 1438. Moreover, an AlGaN/InGN/GaN HFET structure has been disclosed in Japanese Laid-Open Patent Publication No. 2006-32524.
A conventional AlGaN/GaN HFET structure on a sapphire substrate will be described below.
FIG. 10 is a cross section illustrating the conventional AlGaN/GaN HFET structure on the sapphire substrate. In FIG. 10 illustrated are a sapphire substrate 901, an undoped GaN layer 902, an AlGaN electron supply layer 903, a Ti/Al source electrode 904, a PdSi gate electrode 905, a Ti/Al drain electrode 906, and a device isolation layer 907.
In this case, on a (0001) plane of the sapphire substrate 901, a low temperature GaN buffer layer, the undoped GaN layer 902 having a thickness of 2 μm, and the Al0.15Ga0.85N electron supply layer 903 having a thickness of 25 nm are formed in this order by Metal Organic Chemical Vapor Deposition (MOCVD). On a surface of the AlGaN electron supply layer 903, the PdSi gate electrode 905, the Ti/Al source electrode 904, and the Ti/Al drain electrode 906 are respectively provided, and further, the device isolation layer 907 is provided by a selective oxidation.
FIG. 11 shows Ids-Vgs characteristics of the field-effect transistor of FIG. 10. It can be understood that in FIG. 11, the threshold voltage is −2 V, and a normally ON characteristic is illustrated.
FIG. 12 shows Ids-Vds characteristics in the OFF state. It can be understood from FIG. 12 that an OFF state breakdown voltage is 70 V, where the gate length Lg is 1 μm, and the distance between the gate and the drain is 3 μm. In this case, “OFF state breakdown voltage” is defined as a voltage value at a time when the drain current Ids attains 100 μA/mm by varying a voltage applied between the source and the drain, where the gate voltage Vg is −5 V. This definition is valid for hereinafter described field-effect transistors of the present invention.
However, the conventional AlGaN/GaN HFET structure has a low breakdown voltage of 70 V. In order to realize a high breakdown voltage, it is necessary to expand the distance between the gate and the drain, which results in an increased chip area and a high cost. Meanwhile, in case of a high breakdown voltage, there is a problem that an ON resistance, which is a series resistance between the source and the drain in the ON state, increases, causing greater deterioration in a power switching element. Moreover, as described above, since a high sheet carrier concentration due to the polarization electric field results in a threshold voltage of −2 V indicating a normally ON characteristic, it is difficult to realize a normally OFF characteristic, which is highly required for a power switching element.